This invention relates to a method and apparatus for measuring surface roughness and more particularly to a means for replicating the surfaces of ships in an underwater environment.
The role of hull surface roughness in retarding the effective performance of highspeed ships by increasing the frictional drag, and consequently, the operational power requirements is well known. One cause of a gross type of surface roughness is biological fouling of the hull surfaces by calcerous organisms and various algae. Normally, the effects of the highly active biological fouling environment are minimized by use of antifouling paints and periodic cleaning of the hull. Other causes of a gross type of surface roughness include material defects, fabrication errors and poor workmanship which can be substantially minimized by careful manufacturing and fabrication techniques. However, irrespective of the fabrication techniques and biological cleaning procedures, a significant amount of microsurface roughness is inherently present on the painted hull surfaces of ships. This micro or residual roughness, which substantially contributes to frictional drag, is related to the texture characteristics of the paint or coating; the surface preparation and application techniques; erosion, corrosion and pitting of the coating; mechanical damage due to abrasion of the coating; and biological and chemical changes in the coated surface. Since this micro roughness exerts considerable influence on such operational parameters as speed capability, fuel consumption and noise generation, the development and utilization of procedures to determine such micro roughness is especially desirable.
A particular example of a proposed molding apparatus for replicating the minute surface characteristics of the surface of a hull is disclosed in U.S. Pat. application Ser. No. 947,390 filed Oct. 2, 1978, by A. Ticker and H. Preiser. The Ticker molding apparatus generally comprises a cup-shaped housing having an interior chamber partitioned by a diaphragm into a closed-off rear subchamber and a forward subchamber which forms a replicating chamber when the housing is positioned against and secured to the hull surface. A piston member containing a wax molding material is connected to the diaphragm so that upon selective evaucation of the forward subchamber and flooding of the rear subchamber the membrane and piston member are forced toward the hull surface until the molding material is in contact with the hull surface. Thereafter, a resistive heating element, which is connected to the wax molding material, is actuated so that the wax material softens and conforms to the surface of the hull. Upon hardening of the wax material, which may include thermosetting substances, water is admitted to the forward subchamber to force the membrane and the piston member away from the hull so that the wax impression is separated therefrom.
However, molding apparatus employing wax molding materials have an inherent drawback of requiring heating means for supplying a proper amount of heat to soften the wax material without unduly melting the replicating portions of the wax material. Also, since the surfaces to be replicated essentially behave as large heat sinks, it may be especially difficult to maintain portions of the wax material in the appropriate state. Other problems experienced with wax replicating techniques are that air pockets may become trapped between the wax material and the replicated surface and that the wax mold is often difficult to remove intact.